Malloc Recitation By sseshadr

Agenda Macros in C Pointer declarations Casting and Pointer Arithmetic Malloc

Macros

Runtime, compile-time, or pre-compile time? Constant: – #define NUM_ENTRIES 100 – OK Macro – #define twice(x) 2*x Not OK twice(x+1) becomes 2*x+1 – #define twice(x) (2*(x)) OK – Use lots of parenthesis, its a naïve search-and-replace!

Macros Why macros? – Faster than function calls Why? – For malloc Quick access to header information (payload size, valid) Whats the keyword inline do? – At compile-time replaces function calls with code

Pointer declarations

C operators (K&R p. 53) OperatorsAssociativity () [] ->.left to right ! ~ * & (type) sizeofright to left * / %left to right + -left to right >left to right >=left to right == !=left to right &left to right ^left to right |left to right &&left to right ||left to right ?:right to left = += -= *= /= %= &= ^= != >=right to left,left to right Note: Unary +, -, and * have higher precedence than binary forms

Review of C Pointer Declarations (K&R section 5.12) int *p int *p[13] int *(p[13]) int **p int (*p)[13] int *f() int (*f)() int (*(*f())[13])() int (*(*x[3])())[5] p is a pointer to int p is an array[13] of pointer to int p is a pointer to a pointer to an int p is a pointer to an array[13] of int f is a function returning a pointer to int f is a pointer to a function returning int f is a function returning ptr to an array[13] of pointers to functions returning int x is an array[3] of pointers to functions returning pointers to array[5] of ints

Pointer casting, arithmetic, and dereferencing

Pointer casting Separate from non-pointer casting – float to int, int to float – to No! gcc error. Cast from – * to * – * to integer/ unsigned int – integer/ unsigned int to *

Pointer casting What actually happens in a pointer cast? – Nothing! Its just an assignment. Remember all pointers are the same size. – The magic happens in dereferencing and arithmetic

Pointer arithmetic The expression ptr + a doesnt always evaluate into the arithmetic sum of the two Consider: * pointer = …; (void *) pointer2 = (void *) (pointer + a); Think about it as – leal (pointer, a, sizeof(type_a)), pointer2;

Pointer arithmetic int * ptr = (int *)0x ; int * ptr2 = ptr + 1; char * ptr = (char *)0x ; char * ptr2 = ptr + 1; int * ptr = (int *)0x ; int * ptr2 = ((int *) (((char *) ptr) + 1)); void * ptr = (char *)0x ; void * ptr2 = ptr + 1; void * ptr = (int *)0x ; void * ptr2 = ptr + 1;

Pointer arithmetic int * ptr = (int *)0x ; int * ptr2 = ptr + 1; //ptr2 is 0x char * ptr = (char *)0x ; char * ptr2 = ptr + 1; //ptr2 is 0x int * ptr = (int *)0x ; int * ptr2 = ((int *) (((char *) ptr) + 1)); //ptr2 is 0x void * ptr = (char *)0x ; void * ptr2 = ptr + 1; //ptr2 is 0x void * ptr = (int *)0x ; void * ptr2 = ptr + 1; //ptr2 is still 0x

More pointer arithmetic int ** ptr = (int **)0x ; int * ptr2 = (int *) (ptr + 1); char ** ptr = (char **)0x ; short * ptr2 = (short *) (ptr + 1); int * ptr = (int *)0x ; void * ptr2 = &ptr + 1; int * ptr = (int *)0x ; void * ptr2 = ((void *) (*ptr + 1)); This is on a 64-bit machine!

More pointer arithmetic int ** ptr = (int **)0x ; int * ptr2 = (int *) (ptr + 1); //ptr2 = 0x c char ** ptr = (char **)0x ; short * ptr2 = (short *) (ptr + 1); //ptr2 = 0x c int * ptr = (int *)0x ; void * ptr2 = &ptr + 1; //ptr2 = ?? //ptr2 is actually 8 bytes higher than the address of the variable ptr int * ptr = (int *)0x ; void * ptr2 = ((void *) (*ptr + 1)); //ptr2 = ?? //ptr2 is just one higher than the value at 0x (so probably segfault)

Pointer dereferencing Basics – It must be a POINTER type (or cast to one) at the time of dereference – Cannot dereference (void *) – The result must get assigned into the right datatype (or cast into it)

Pointer dereferencing What gets returned? int * ptr1 = malloc(100); *ptr1 = 0xdeadbeef; int val1 = *ptr1; int val2 = (int) *((char *) ptr1); What are val1 and val2?

Pointer dereferencing What gets returned? int * ptr1 = malloc(sizeof(int)); *ptr1 = 0xdeadbeef; int val1 = *ptr1; int val2 = (int) *((char *) ptr1); //val1 = 0xdeadbeef; //val2 = 0xffffffef; What happened??

Malloc

Malloc basics What is dynamic memory allocation? Terms you will need to know – malloc/ calloc / realloc – free – sbrk – payload – fragmentation (internal vs. external) – coalescing Bi-directional Immediate vs. Deferred

Fragmentation Internal fragmentation – Result of payload being smaller than block size. – void * m1 = malloc(3); void * m1 = malloc(3); – m1,m2 both have to be aligned to 8 bytes… External fragmentation

Implementation Hurdles How do we know where the chunks are? How do we know how big the chunks are? How do we know which chunks are free? Remember: cant buffer calls to malloc and free… must deal with them real-time. Remember: calls to free only takes a pointer, not a pointer and a size. Solution: Need a data structure to store information on the chunks – Where do I keep this data structure?

The data structure Requirements: – The data structure needs to tell us where the chunks are, how big they are, and whether theyre free – We need to be able to CHANGE the data structure during calls to malloc and free – We need to be able to find the next free chunk that is a good fit for a given payload – We need to be able to quickly mark a chunk as free/allocated – We need to be able to detect when were out of chunks. What do we do when were out of chunks?

The data structure It would be convenient if it worked like: malloc_struct malloc_data_structure; … ptr = malloc(100, &malloc_data_structure); … free(ptr, &malloc_data_structure); … Instead all we have is the memory were giving out. – All of it doesnt have to be payload! We can use some of that for our data structure.

The data structure The data structure IS your memory! A start: – – What goes in the header? Thats your job! – Lets say somebody calls free(p2), how can I coalesce? Maybe you need a footer? Maybe not?

The data structure Common types – Implicit List Root -> chunk1 -> chunk2 -> chunk3 -> … – Explicit List Root -> free chunk 1 -> free chunk 2 -> free chunk 3 -> … – Segregated List Small-malloc root -> free small chunk 1 -> free small chunk 2 -> … Medium-malloc root -> free medium chunk 1 -> … Large-malloc root -> free large chunk1 -> …

Design considerations I found a chunk that fits the necessary payload… should I look for a better fit or not? Splitting a free block: void* ptr = malloc(200); free(ptr); ptr = malloc(50); //use same space, then mark remaining bytes as free void* ptr = malloc(200); free(ptr); ptr = malloc(192);//use same space, then mark remaining bytes as free??

Design Considerations Free blocks: address-ordered or LIFO – Whats the difference? – Pros and cons? Implicit / Explicit / or Seg? – Implicit wont get you very far… too slow. – Explicit is a good place to start, and can be turned into a seg-list. – Seg-list: what are the thresholds?